USB: f_rndis: Check if cdev is NULL before accessing

RNDIS control path completion handlers are getting called after setting
cdev to NULL during disconnect as part of composition switch and this
is leading to a crash. Avoid this crash, by checking, if cdev is NULL
before accessing. Also don't queue request's to DCD, if composition switch
is in progress.

(cherry picked from commit 46874732be9d6997ff3bceaf97f07911f9f22440)

Change-Id: If4f1f2f95d624c34733ef29febed17950da43b0e
CRs-Fixed: 433534
Signed-off-by: Chiranjeevi Velempati <cvelempa@codeaurora.org>